For many years the ability to communicate at frequencies at for instance between 10 Hz and 1000 Hz has been important primarily for communication with submarines in which extremely low 10 Hz-2000 Hz signals are required to penetrate to the ocean depths or, for instance for finding underwater objects or doing underground exploration.
There is a problem with ground antennas for ELF/ULF communication in that conventional antennas are very inefficient because while they project energy into free space, they also drive a return current of equal intensity inside the ground. The return current radiates an E field that partially cancels the E field in both the submarine receiver as well as in the ionosphere above the ELF transmitter. As a result the received signal strength is weaker by the effect of the ground return currents. The result in terms of radiated power is that radiated signals are 40 dB or more below that which could occur if one did not ground the return problem.
In the past in order to establish extra low frequency, ultra low frequency or very low frequency communications, it has been found that by projecting high frequency, HF signals into the ionosphere the HF signal is converted by an electrojet current created by the solar wind into an ELF/ULF signal which for instance is capable of penetrating below the surface of the ocean to a depth for instance of 500 feet or more.
The HAARP project, or the High Frequency Active Auroral Research Program, was instigated to investigate how the ionospheric layers could be affected or in fact activated utilizing High-Frequency (HF) electromagnetic waves in the 2.8-10 MHz range produced by a ground-based transmitter.
The initial theory for providing low frequency communication was to in essence create a virtual ELF antenna in the lower ionosphere. To provide such a radiating capability so-called electrojets were utilized. These electrojets are electric currents that run horizontally in the ionosphere and are created by the interaction of the solar wind with the earth's magnetosphere. When HF signals modulated at the desired ELF frequency interact with the electrojet a virtual antenna is generated at the modulated frequency, essentially down-converting to the HF to ELF frequency. The down converted energy propagates in a wave guide established between the ionosphere and ground.
While the injection of HF energy into the ionosphere when interacting with an electrojet produces a significant amount of low frequency energy, the electrojet is not present all the time. One can go to three to five days without having any electrojet current. Moreover, even when the electrojet exists, the distance from where one wants to send signals to the point of receipt is on the order of 4 to 5 mega meters or approximately 3000 miles, and one can lose 20 to 30 dB in received signal strength simply by virtue of the distance and attenuation involved.
Thus in the HAARP scenario, one sends amplitude modulated radio-waves towards the ionosphere. The radio-waves modulate the conductivity of the ionospheric plasma by periodically heating the ionospheric electrons and thus creating an oscillatory current. It is this current that radiates in the low frequency domain sufficiently to provide subsurface communications.
Prior to the HAARP experiments, the only technique to communicate with subsurface vessels such as submarines was to utilize extra low frequencies, for instance at 76 hertz and to project these electric fields into a wave guide formed by the ionosphere and the surface of the earth. These types of communication were provided by a NAVY communications facility in the state of Michigan. At a frequency of 76 hertz the wavelength corresponds to 4000 kilometers. It is noted that the efficiency of an antenna is inversely proportional to the wavelength, and with these wavelengths the antenna is totally inefficient. While the NAVY facilities in Michigan did not employ 4000 kilometer antennas, they did deploy antennas of about 50 kilometers.
These shortened antennas further complicated the efficient transfer of energy to a subsurface vessel and because of the lack of signal strength it was common practice to order the submarines to the surface so that they could receive signals. Typically it would take more than ten minutes to tell a submarine to surface and prepare to receive signals at a different frequency (e.g. 20 kHz) which engendered discovery of their location.
Thus there is a requirement to be able to reliably communicate with subsurface vessels or to detect subterranean objects and to provide a system which is all-weather and does not rely on the vagaries of solar wind.
As mentioned before, the problem with low frequency signals is the effect of ground loop cancellation.
The HAARP project was originally designed to reduce the effect of ground currents by lifting up the antenna above the ground. The result is that since the ground loop current effect is proportional to distance of the antenna from the ground, the further up that one can provide a virtual antenna, the less affected it will be by the ground currents. Noting that the ionosphere starts at 80 kilometers, the ability to provide the aforementioned electrojet based antennas was effective to substantially reduce the effect of the ground currents.
However, since electrojets generated by the solar wind are unreliable one was faced with having to solve the low frequency communications problem in another way.